In general, the process for commercially producing glass is broadly classified into a melting step, a refining step and a forming step, and feeding equipment is disposed between steps as required. The melting of glass is done by supplying various kinds of powder materials in a furnace comprising refractories and by melting the supplied powder materials at a high temperature. In order to improve the quality and the homogeneity of such molten glass, it is inevitable to dispose a refining zone, such a sub-atmospheric apparatus.
As such a sub-atmospheric apparatus, has been disclosed the one as shown in FIG. 10 (see, e.g., JP-A-9-142851).
Specifically, the sub-atmospheric apparatus 10 shown in this figure includes an upstream transferring canal 30A for supplying molten glass 21, an uprising pipe 22U for sending the molten glass 21 vertically and upward at a downstream end of the upstream transferring canal 30A, a sub-atmospheric vessel 20 disposed so as to extend substantially horizontally from an upper end of the uprising pipe 22U, a downfalling pipe 22L for sending the molten glass 21 vertically and downward from a downstream end of the sub-atmospheric vessel 20, and a downstream transferring canal 30B for further directing the molten glass 21 in a downstream direction from the downfalling pipe 22L.
The upstream transferring canal 30A includes a first stirrer 31a, and the downstream transferring canal 30B includes a second stirrer 31b. 
The uprising pipe 22U, the sub-atmospheric vessel 20 and the downfalling pipe 22L are covered by casings 23.
The uprising pipe 22U, the sub-atmospheric vessel 20 and the downfalling pipe 22L are formed in an arched shape as a whole and serve to pump the molten glass 21 to the sub-atmospheric vessel 20 on the siphon principle and to utilize a pressure difference to remove bubbles contained in the molten glass 21.
Each of the sub-atmospheric vessel 20, the uprising pipe 22U and the downfalling pipe 22L comprises precious metal or the like in order to avoid reaction with the molten glass 21.
By this arrangement, the molten glass 21, which is being supplied from the upstream transferring canal 30A, is stirred by the first stirrer 31a on the way to the uprising pipe to change a dissolved gas into tiny bubbles and make the molten glass 21 uniform. The molten glass 21 is pumped to the sub-atmospheric vessel 20 through the uprising pipe 22U and is degassed in the sub-atmospheric vessel 20. The degassed molten glass 21 is directed to the downstream transferring canal 30B through the downfalling pipe 22L and is transferred to the forming step.
The most important property for the material of parts in direct contact with the molten glass 21 in the sub-atmospheric vessel 20 stated earlier is to prevent glass from being contaminated.
A similar required characteristic is also required for the material of the equipment for connecting respective melting, refining and forming zones to transfer the molten glass.
From this viewpoint, specific precious metal having a high melting point has been frequently utilized in the equipment, which deals with molten glass. In particular, when producing a glass product required for having functionality, it is more strongly encouraged to decrease the amount of impurities, which are introduced from the material of the parts.
However, it is not acceptable to use such precious metal in a large amount as in usual ferrous or nonferrous metal since such precious metal is extremely expensive. For this reason, the precious metal that is used in glass producing equipment has been formed in a thin plate to be utilized as a refractory structure lining or to be utilized as a thin cylindrical pipe having a complete round cross-section, which is the most difficult to collapse.
The arrangement wherein a refractory structure, such as the sub-atmospheric apparatus 10, is lined with precious metal as stated earlier has a problem of mismatching in thermal expansion between refractory and precious metal. For this reason, a thin cylindrical pipe made of precious metal and having a complete round cross-section is utilized.
However, it is inevitable to increase the diameter of such a cylindrical pipe to enlarge the cross-sectional area of the pipe since the amount of the molten glass 21 to deal with tends to increase recently. Several problems stated below have become evident because of such a demand to increase the diameter.
First, there is a problem that a mere increase in the diameter means a proportional increase in the amount of precious metal to use, which results in an explosive increase in equipment costs.
Next, there is a problem of glass defect, which is caused by the sublimation phenomenon of precious metal. Although this problem is rarely caused when molten glass is filled in a cylindrical pipe without gaps, this problem cannot be ignored since the cylindrical pipe is actually used, having spaces remaining therein, in a few cases.
In other words, a slight amount of oxide is caused to sublimate from a surface of precious metal when the precious metal is exposed to a high temperature of a thousand and several hundreds ° C. or above in the presence of oxygen in a sufficient amount. The oxide thus formed is not stable, and the oxide is reduced. by a subtle environmental change, causing tiny metal particles to be recrystallized. Tiny metal particles thus recrystallized are taken in the molten glass. Since the tiny metal particles in the molten glass are not completely dissolved, some of the particles are solidified and remain in the glass, causing quality defect.
The present invention is proposed in consideration of the problems stated earlier. It is an object of the present invention to provide a conduit for molten glass, a molten glass degassing method and a molten glass degassing apparatus, which are capable of producing homogenous and good quality glass at a low cost.